The murine retroviruses are of interest both because of the pathology of the diseases which they induce, and because of their utility as general models of enveloped viruses. Many biologically important functions of these viruses reside in the viral env gene products. The overall objectives of our study are to determine structural-functional correlations for the env proteins of different classes of MuLV. We have to date developed procedures for isolating fragments of gp70 and p15(E) which correspond to specific structural domains of these proteins. More detailed studies of the primary, secondary, and tertiary structure of these domains will be performed, including analyses of the oligosaccharide constituents at specific glycosylation sites, localization of disulfide bonds, and studies of the topographical organization of these domains in single molecules and in oligomeric complexes of the env proteins. The contribution of the individual domains to such functional properties of the env proteins as receptor binding, cell fusion, and sensitivity to serum neutralizing factor, will be determined both by direct studies with the isolated subunits, and indirectly with antibodies directed against specific domains. The function of various structural regions and features of the env gene products in the processing, assembly, and biological activities of these proteins will be analyzed by in vitro site-specific mutagenesis of cloned viral DNAs, using synthetic oligonucleotides to introduce precise amino acid changes into selected regions of the env gene products. Cells expressing the mutant viruses will be isolated by cotransfection with a selectable gene, and the structural, immunological, and functional properties of the mutated gene products will be determined. In this way, the functional roles of hydrophobic leader and transmembrane sequences, conserved glycosylation sites and cysteine residues, and proteolytic cleavage sites will be determined.